Immunoreactive substance P (I-SP) was measured in two pheochromocytomas and a carcinoid of the caecum. The pheochromocytomas contained 1,5 and 50 pmol/g I-SP respectively. An HPLC analysis I-SP and catecholamines were enriched in the chromaffin granule fraction. The carcinoid contained 2900 pmol/g I-SP. About 40% of the immunoreactivity behaved like substance P on gel filtration, HPLC and in bioassays (fall in rabbit blood pressure and contraction of the guinea-pig ileum which was abolished by specific tachyphylaxis to substance P). These results indicate that authentic substance P is present in carcinoids and pheochromocytomas and that in the latter it is most likely stored in chromaffin granules.